The Open Accessible Space Information System (OASIS) project began in 2000 as a way for the USDA Forest Service to map the open spaces of New York City. The first part of the project saw almost 40 different groups of educators, green groups, businesses, and agencies gathering and sharing data to create a huge geo-data mashup. The maps have been recently updated and the project is excellently summarised in a post at the Urban Omnibus.

The maps are used by many different groups to visualise transport routes, green open spaces, land use, population data, leisure centres, wildlife areas, school districts, and countless other data sets about the built and natural environment of the city of New York.

Open Street Map is a similar initiative, but there is not as much shared data on this site, as yet. Greenmaps is a open source sustainable mapping and social networking tool (London map not as yet published). MapTube allows users to overlay different maps of London including those covering population, transport, and crime.

The Londonist has a good list of user-created and alternative maps of London including maps of historical places, food markets, and transport details. One such map, which is of interest to me as I am about to move to London, is Where-can-I-live. You type in where you need to commute to, how much time you want to spend commuting, how much rent you can afford, and the site shows you the best places to live.